HISPEC/DESPEC Executive summary
The FAIR-NUSTAR facility will provide beams of radioactive ions with unprecedented intensities with the aim to study the atomic nucleus.
We will get information on the force acting between the nucleons inside the nucleus, with special emphasis on systems with exotic proton-to-neutron ratios: both proton rich and neutron rich nuclei. In extreme neutron-rich nuclei radical changes in their structure are expected with the possible disappearance of the classical shell gaps and magic numbers and the appearance of new ones. They are also important for nuclear astrophysics. HISPEC/DESPEC addresses this kind of questions using radioactive beams delivered by the energy buncher of the Low Energy Branch (LEB) of the Super Fragment Separator with energies of 3-150 MeV/u for reaction studies or stopped and implanted beam species for decay studies.
The project focuses on those aspects of nuclear investigations with rare isotope beams which can be uniquely addressed with thigh-resolution setups.
DESPEC (DEcay SPECtroscopy)
Decay studies lie at the very frontier of the field of exotic nuclei, since once the existence of an isotope has been demonstrated, the next elementary information we seek is how it decays, even an imprecise number on the half live of a new isotope can tell us a lot about the allowed or forbidden character of the decay. At the same time decay spectroscopy provides often primary information on excited states of nuclei far from stability. The advantage of the decay experiments is that they can be based on a relatively small number of events. A unique feature of the FAIR Super-FRS will be the access to regions where the waiting points for the r-process occur. For our understanding of the r-process nucleo-synthesis of heavy elements in supernova explosions we need to know the beta decay half life, the neutron branching ratios and the neutron (or two-neutron) separation energy of these nuclei. At the DESPEC set up we will be able to measure the first two quantities while the last will be measured either at ILIMA or at MATS. If the number of decays is sufficiently high, detailed spectroscopy will be possible and then questions such as isospin symmetry can be tested in mirror nuclei or the long standing Gamow Teller quenching problem in beta decay can be addressed in combination with charge exchange reactions performed at R3B or EXL. On a more fundamental level superallowed Fermi transitions in odd-odd N=Z nuclei can be used to explore issues such as the unitarity of the CKM matrix in the Standard Model of electroweak interactions. For the most exotic nuclei we can expect some unusual decay modes such as beta-delayed multi-neutron emission, beta delayed fission, or even direct neutron radioactivity. Another very important aspect of DESPEC is the possibility to study the decay properties of isomeric levels in nuclei which survive the flight time from the moment of production until the time of arrival to our set-up.
All of the experiments anticipated at DESPEC involve deep implantation of the ions in an active stopper prior to the decay. The detector will be highly pixellated, which allows us to correlate in time and space the signal of the initial pulse from implantation of the heavy ion with the signal produced in the same detector in the subsequent beta decay. Neutron and high resolution gamma-ray detectors in a compact arrangement around the active stopper in a highly flexible and modular geometry will be at the heart of this set-up. Complementary measurements using the Total Absorption Gamma technique and measurements of nuclear g-factors and quadrupole moments as well as level half lives are also foreseen.
HISPEC (HIgh-Resolution In-flight SPECtroscopy)
In the previous paragraphs we have described the physics to be addressed at the radioactive decay set-up, the other fundamental pillar to study the properties of the nuclei is by means of nuclear reactions. They have the advantage of their high flexibility. By selecting a suitable combination of projectile target, and beam energy, we can obtain a variety of results ranging from the reaction products to the character of the states that are populated. At the HISPEC set-up these kind of studies can be carried out with radioactive beams of intermediate energies, as delivered by the energy buncher of the Super-FRS or at energies around the Coulomb barrier with further decelerated beams. Single step Coulomb excitations and fragmentation reactions at intermediate energies as well as inelastic scattering, transfer reactions and fusion evaporation reactions at lower energies will provide information about transition probabilities, single particle spectroscopic factors, high spin states, etc. By observing the single particle or collective vibrational or rotational character of the states we can conclude about basic properties of the nucleus such as the shape. These reactions are often complementary to those described at R3B or EXL. The advantage here is that one can use high resolution Ge detectors to measure the gamma de-excitation of the levels populated. The HISPEC set-up has at its core AGATA, the next generation γ-ray tracking array, with a resolving power hugely exceeding the presently available Ge-arrays. In addition, the setup will comprise beam tracking and identification detectors placed before and behind the secondary target, charged particle detectors, a plunger, a magnetic spectrometer and other ancillary detectors.
HISPECandDESPEChave much in common in terms of both the physics and the instrumentation, for instance they will use the same suite of ion identification and tracking detectors. There is also a large overlap in terms of the community involved and in a very natural way, they have decided to join forces. The two set-ups will be combined for specific recoil decay experiments, with the DESPEC detectors placed at the end of the magnetic spectrometer.